This invention relates to non-invasive ultrasonic measurement of properties of materials by a pulse echo technique, as in seismology, sonar, and medical imaging. More particularly, the present invention relates to the production of a two dimensional image of the density of physiological material when a sector scan or other type of two dimensional scan is used for medical diagnosis.
Pulse echo techniques for non-invasive determination of the properties of materials have been developed for many fields, including radar, sonar, seismology, and medical diagnostic ultrasound. The following discussion will relate specifically to the last of these, and even more specifically to a system intended to produce an image of a plane section by means of a raster scan. A typical sector scan geometry is shown in FIG. 1. Component 1 comprises the bulk of the ultrasound system, including the final display or print out device, and 2 is the ultrasonic transducer. Element 3 is the physiological system being scanned. For the sake of simplicity, it will be assumed that 3 consists of two volume regions, 3a and 3b, with differing parameters, but that the parameters are uniform within each region. To produce a two dimensional planar image, a number of closely spaced line scans are made sequentially, in a plane, by slightly changing the angle of the transducer between successive lines. The result shown in FIG. 1 is a sector scan, but other types of raster scan are also possible.
The process of producing a signal for each line in turn is identical, and the following discussion will pertain to the single line containing points A and B. The portion of the system shown as 1 in FIG. 1 is shown in more detail in FIG. 2. A pulse generator, 4, generates short excitation pulses spaced farther apart in time than the time required for all reflections from one pulse to arrive back at the transducer. The pulses pass through a transmit/receive switch, 5, to a transducer, 6, which converts the electrical pulse to an acoustic pulse which is directed into the material under study. The acoustic pulse, suitably focused, propagates through the material in approximately a straight line, governed by the wave equation. There is attenuation with distance, but no reflection so long as the properties of the material are uniform. For liquids, the density and one elastic constant, compressibility, determine the main features of the propagation. Although seeming to be a considerable over simplification for most physiological systems, analysis with these parameters is often reasonably satisfactory. A better model for anisotropic solids requires another elastic constant, shear modulus. For isotropic solids, such as muscle, more elastic constants are required, up to a theoretical total maximum of 21. In all cases, another constant representing a dissipation factor, determines the attenuation.
Exact analysis is virtually impossible with the complex geometry encountered in physiological imaging, but fortunately it is unnecessary because of two general considerations. The first is that, irrespective of the number of parameters, when any change in the value of density or elastic constants is encountered, as at point A, a fraction of the incident energy will be reflected back toward the transducer, where it is converted back to an electrical signal. This proceeds through the transmit/receive switch to an amplifier, 7, which uses programmed or automatic gain control, or both, to maintain the amplitude of reflections from identical changes in parameters at different depths approximately constant despite the effect of attenuation. A detector, 8, senses the envelope of the reflected pulse and it is stored and/or displayed as a function of the time following the excitation pulse. Only a display is shown in FIG. 2. The second general consideration is that the change in velocity caused by the change in material parameters that give rise to useful reflections is fairly small, so long as there is no bone or air in the path. Consequently, since the approximate velocity is known, the time between the excitation and the return of a reflection can be interpreted as being proportional to the distance from the transducer to the point at which a change in parameters caused the reflection. Reflections from deeper points, such as B, arrive at later times, and the complete set of reflections from one pulse, two in this example, result in the signal shown on the display, 9. Similar displays for the other scan lines, properly positioned on the display, provide an outline of region 3b. For the sake of simplicity, changes in material parameters will henceforth be referred to as changes in density, although changes in elastic constants may also contribute. The image, although resulting from changes in density, is not an image of density versus position. Rather, it is an image of the absolute value of the derivative of density in the direction of propagation of the ultrasonic wave. For most purposes an image of density would be more desirable, but it cannot be obtained by integrating along the direction of propagation. That is because the value of the reflection, including polarity, would have to be integrated to provide density, but the envelope detection gives only the absolute value of the derivative.